Low voltage spark plugs

ABSTRACT

A low voltage spark plug wherein the sparking end of the center electrode positioned within the spark plug insulator is provided with a cylindrical bore, a conically-shaped electrode electrically connected to the shell at one end and positioned within the bore with its axis substantially coincident with that of the bore and with the point of the cone positioned within the bore.

United States Patent Cataldo {4 1 Apr. 25, 1972 [54] LOW VOLTAGE SPARK PLUGS FOREIGN PATENTS OR APPLICATIONS [72] Inventor: Roy S. Cataldo, Birmingham, Mi h, 19,800 1911 Great Britain ..3l3/139 [73] Assignee: General Motors Corporation, Detroit, Examiner Nathan K f a Mich. Attorney-Sidney Carter and Peter A. Taucher [22] Filed: May 22, 1970 ABSTRACT [21] Appl' 39850 A low. voltage spark plug wherein the sparking end of the center electrode positioned within the spark plug insulator is 52 US. Cl ..313/141 313/139 Pmidel with a cylindrical a e'wmde [51] Int CL 6 13/20 electrically connected to the shell at one end and positioned 58] Field l 41 142 within the bore with its axis substantially coincident with that of the bore and with the point of the cone positioned within 313/336 the bore.

[56] References Cited 1 Claims, 4 Drawing Figures UNiTED STATES PATENTS 971,908 10/1910 Low ..313/141 X Patented April 25, 1972 3,659,137

SPARK PLUG SFARKING POTENTIAL DATA 0 HOURS LEGEND I o 1000 RPM IZSOORPM I3 -A4000RPM x 600 RPM LEGEND STANDARD PLUGS O T000 RPM II 02800 RPM A4000 RPM a 6OORPM ,2 g CONBQYLINDER PLUGS LOAD LB-FT 7 SPARK PLUG SPARKING POTENTIAL Z Z DATA I25 HOURS 9' r LL0LND LEGEND o|000 RPM 0 1000 RPM I4 2800 RPM 02800 RPM H A4000 RPM 4000 RPM X 600 RPM 81 600 RPM g STANDARD PLucs CONE-CYLINDER PLUGS- ll IO L19 5O LOADQFB-FTM I 9.4

' fiiyiCazfala o ATTORNEY INVEN'IOR.

LOW VOLTAGE SPARK PLUGS Low voltage spark plugs have always been sought after in connection with internal combustion engine usage in order to minimize the effect of sparking erosion on the electrodes and achieve a lower temperature at the sparking tip in order to obtain longer life and enable the use of relatively low cost ignition components. The common method of achieving such low voltage has been the provision of a creep-gap design achieved by forming the tip of the insulator adjacent the spark gap with a semiconductor material in order that the spark might move across the surface thereof between the electrodes rather than jump the air gap therebetween as when a standard insulator tip is used. Typical of such creep gap design is US. Pat. No. 2,894,315 issued July 14, 1959 to Alfred Candelise.

Another advantage of low voltage plugs is that they tend to operate at lower temperatures. Typical of high voltage-low temperature plugs is US. Pat. No. 2,391,459 issued to Hensel Dec.25, 1945.

Such prior art low voltage plugs have been relatively expensive in manufacture and have not had entirely desirable life characteristics. In accordance with the spark plug design of my invention, low voltage characteristics are achieved without special coatings and a minimum of relatively expensive erosion resistant electrode material may be used. As is more fully described hereinafter, the design of my invention permits the use of conventional spark plug design except as to the electrode configuration at the sparking end of the plug.

It is an object of my invention to provide a low voltage spark plug wherein the relatively low sparking potential is achieved by means of electrode geometry.

It is another object of my invention to provide a spark plug having a low sparking potential wherein a minimum of relatively high cost erosion resistant material is utilized.

A further object of my invention is to provide a spark plug having a low sparking potential and capable of meeting Federal standards covering radiated interference.

These and other objects of my invention are achieved by providing a cylindrical bore at the firing end of the center electrode and by providing a conically-shaped electrode secured at one end to the shell of the spark plug with its axis substantially co-incident with that of the cylindrical bore and with the tip of the cone positioned therein.

The foregoing and'other objects of my invention are more fully described in the following description and attached drawing wherein:

FIG. 1 shows a cross section of a preferred embodiment of the spark plug of my invention;

FIG. 2 shows a broken away enlarged sectional view of the firing end of the spark plug of my invention with the cone point between both ends of the cylindrical bore;

FIG. 3 shows a plot of the sparking potential of both a standard automotive plug and a plug in accordance with my invention, varying with load and with engine speed, the data being taken on new plugs;

FIG. 4 discloses a similar plot after each type of spark plug has undergone 125 hours of operation in a motor vehicle.

As shown in FIG. 1, the spark plug of my invention comprises standard type design up to the sparking tip. More specifically, spark plug 1 comprises a metal shell 3 within which there is concentrically positioned an insulator body 5 having a centerbore 7. The insulator 5 is sealed within the shell 3 in the conventional manner using top and bottom metal ring seals 9 positioned on the shoulders of the insulator body. Sealed within centerbore 7 and in electrical interconnection are the terminal screw 11 and the center electrode 13.

In accordance with my invention, the sparking end of the center electrode 13 is provided with a cylindrical bore 15 within which there is positioned a conically-shaped electrode 17 having one end secured to the shell 3, the point of the cone being positioned within the bore 15. As is more clearly shown in FIG. 2, the axis of both the bore 15 and the conicallyshaped electrode 17 are substantially co-incident. As is also more clearly shown in FIG. 2, the preferred embodiment of my invention comprises the use of a washer-like metal member 19 having an outwardly turned tab 21 within which the non-conical end of the electrode 17 is secured in any suitable manner as by pressing, welding, etc. The washer 19 is secured within the firing end of the spark plug shell 3 by crimping the end of the shell over the end of the washer. Other suitable and well known methods of attachment may of course also be utilized. In this regard, it may be specifically noted that the conically-shaped electrode 17 may be directly secured to the shell in any suitable manner as by welding. It should also be understood that whereas the cylindrical bore is preferably provided directly in the end of the center electrode 13, a hollow cylindrical member may itself by secured on the end of the center electrode in any suitable manner such as by welding.

In operation, the electrode design of my invention serves to provide a high electrical field intensity in the area of the point of the electrode 17 thus serving to initiate sparking at a lower voltage than is otherwise obtainable. l have found that the design of my invention tends to be relatively insensitive to erosion due to sparking in view of the fact that once initiated, sparking tends to raise the temperature about the electrode and ionize the surrounding gases thus enabling the primary sparking to occur in an area back from the point and around the electrode 17 with erosion taking place symmetrically around the axis of the electrode thus maintaining the point substantially on the centerline of the cylindrical bore and of the electrode.

1 have also found that longer life is achieved for the conical electrode by using a smaller cone angle. A smaller cone angle is in turn possible for any given diameter of electrode 17 if the cone is designed with the point thereof penetrating the cylindrical bore as far as possible while still being within the bore. Similarly, a smaller cone angle is achieved for any given penetration of the point by using a smaller diameter electrode. By way of example, I have found that a suitable cylindrical bore dimension is about 0.075 inches whereas the conical electrode diameter may be about 0.036 inches in diameter. The center electrode diameter in such case would be about 0.110 inches. It is thus apparent that the conically-shaped electrode 17 is needle-like in configuration.

A further advantage of my invention is that the amount of material represented in electrode 17 is relatively small compared with the erosion resistant material used in conventional spark plug designs. Thus, since the erosion resistant metals are relatively expensive, being formed normally of nickel or barium-nickel alloy or of tungsten and tungsten-nickel alloy, it is obvious that the design of my invention enables the use of a minimum amount of such expensive materials. Similarly, I have found that by reason of the low voltage properties of the cone-cylinder design of my invention it is no longer necessary to form the relatively large'center electrode of the wear resistant materials and that instead this element may be formed of such low cost materials as silicon iron.

I have also found that the effect of wear on the center electrode is also reduced by reversing the polarity of the electrodes so as to have the conically-shaped electrode act as the cathode which will go negative with respect to the center electrode. It should be noted that forming the cathode of a lowwork function material such as nickel or barium nickel enables the electrons to be driven out into the gap with a minimum of energy.

Operating experience in a motor vehicle using both standard type spark plugs and spark plugs in accordance with my invention has demonstrated a lower sparking voltage for the plugs of my design. Such tests were run over a period of hours and for approximately 7,000 miles. FIGS. 3 and 4 show the test results at three speed levels and at idle speed for each type of plug at varying load conditions. FIG. 3 represents the data with new spark plugs whereas FIG. 4 represents the data .after 125 hours of operation. It can be seen that though the sparking potential for each of the type plugs tend to generally parallel each other, the sparking potential for the plugs of my invention are appreciably lower than that for the standard type plugs. More specifically; the data at zero hours shows the sparking voltage for the cone-cylinder design at 1,000 rpm and at 50 lb./ft. loading to vary from about 37 percent of the voltage for the standard design plug to about 58 percent at 150 lb./ft. loading. After 125 hours the same test points show the sparking voltage to vary from about 60 to about 76 percent of that for the standard type plugs.

It may be noted that the sparking voltage for each type plug tends to drop off as thespeed of the engine increases. This is due to the fact that the air-gas mixture is leaner at the lower speeds. Since oxygen is known to be more electronegative than the remaining constituents in the air-fuel mixture, the oxygen removes electrons from the gap area to a greater extent at lower speeds than at high speeds.

l have also found that the spark plug of my design compares favorably with the standard high resistance spark plug from the standpoint of compliance with Federal regulations limiting the amount of radiated interference from automotive ignition systems. Spark plugs in accordance with my invention fall within the limitations of such regulations without the need for a resistor element formed within the centerbore of the plug.

It is apparent from the foregoing description that I have provided a low voltage spark plug which may be readily fabricated in mass production at the same time that relatively small amounts of high cost erosion resistant materials are necessary for electrode fabrication. While I have disclosed a preferred embodiment of my invention, the scope of my invention is as defined in the claim which follows.

What is claimed is:

l. A low voltage spark plug comprising a metal shell having a ceramic insulator positioned therein, a centerbore through said insulator having a terminal screw therein at one end and a center electrode therein at the other end, said terminal screw and electrode being electrically interconnected, a cylindrical bore of 0.075 inches in diameter provided at the sparking end of said center electrode which has a diameter of 0.1 10 inches, a conically-shaped electrode of 0.036 inches in diameter having a connection at one end with said shell andvthe other end positioned with the point thereof within said cylindrical bore, the axis of said conical electrode being in coincidence with that of said bore and said conical electrode extending from one end to the other end of said cylindrical bore and thus producing as small a cone angle as is possible to achieve maximum electrode life. 

1. A low voltage spark plug comprising a metal shell having a ceramic insulator positioned therein, a centerbore through said insulator having a terminal screw therein at one end and a center electrode therein at the other end, said terminal screw and electrode being electrically interconnected, a cylindrical bore of 0.075 inches in diameter provided at the sparking end of said center electrode which has a diameter of 0.110 inches, a conically-shaped electrode of 0.036 inches in diameter having a connection at one end with said shell and the other end positioned with the point thereof within said cylindrical bore, the axis of said conical electrode being in coincidence with that of said bore and said conical electrode extending from one end to the other end of said cylindrical bore and thus producing as small a cone angle as is possible to achieve maximum electrode life. 